The present invention relates to improved electronic circuitry for accurately measuring the width of pulses having widely varying amplitudes, rise times and wave shapes. More particularly, the invention is related to such a circuit for use in an electro-optical label reading system.
In U.S. Pat. No. 3,225,177 entitled MARK SENSING, there is disclosed a system for the electro-optical "reading" of labels on objects for purposes of object indentification. A system embodying the invention of this patent is marketed by the Servo Corporation of America of Hicksville, New York under the trademark KARTRAK.
Briefly stated, the system utilizes pulses resulting from the reflection of a beam of light onto a striped, color-coded label to produce a train of digital pulses indicative of the color-coded label. The width of each pulse is representative of the width of a corresponding stripe of the label scanned while the amplitude of each pulse varies as a function of the physical condition of the label. Generally speaking, the dirtier a label is, the weaker the resultant pulse.
The KARTRAK system referred to above is approved by the American Association of Railroads for railroad car identification. Accordingly, virtually every freight car in the country is provided with a KARTRAK label. These labels are exposed to a wide variety of environmental conditions and, as a result, their physical conditions vary widely. Thus, on any particular train, it would not be uncommon to have a few cars with virtually brand new labels and other cars with labels the stripes of which are barely discernible.
As a result of the above, it was recognized early in the development of the KARTRAK system that means for standardizing the output signals from the label scanner would have to be provided. The object of the standardizer is to provide an output pulse for each input pulse generated from a scanned label stripe with the amplitude of the output pulse being constant and the width being related to the width of the scanned pulse at the 50% amplitude value. Such a standardizer is disclosed in U.S. Pat. No 3,299,271 for ELECTRO-OPTICAL LABEL READING SYSTEM USING PULSE WIDTH DETECTION CIRCUIT.
As the above referenced patent (U.S. Pat. No. 3,299,271) points out (beginning at column 1, line 49) its design anticipates a variance in the amplitude of the returned optical signal over a 100:1 range. While this range was acceptable at the time the invention disclosed in this patent was made, it is no longer acceptable. This is because many of the older automatic car identification labels in actual use in the field for extended periods have gotten extremely dirty. New cars and cars with damaged labels have been provided with new labels which are clean. The dirty labels exhibit a very diminished optical response to the automatic car identification scanners while the very clean, new labels exhibit a relatively large response. As a result, it has been found that a dynamic range on the order of 1,000:1 -- an improvement of 10 times that of the previously disclosed system is now required.
In view of the above, it is the principal object of the present invention to provide an improved standardizer circuit for an electro-optical label reading system capable of accurately producing constant amplitude output pulses of a width corresponding to the 50% amplitude pulse width of input pulses having amplitudes varying over a range on the order of 1,000:1.